The evolution of digital communications has been made possible by advancements in integrated circuitry. Various components of a complete digital voice and data switching system have been implemented in integrated circuit form.
In a conventional digital time division switching matrix, each of N input lines is a time division multiplexed link consisting of a continuous serial data stream subdivided into M serial words, or "time slots". Each group of time slots constitutes one frame. Standards developed by the Bell System and the International Telegraph and Telephone Consultative Committee (CCITT) have resulted in 8 bit time slots and 125 microsecond frame times (8 KHz voice sampling) being used in telephone systems throughout the world. The CCITT standard, for example, provides for 32 time slots or channels (256 bits) per frame. Thirty of the channels are used for voice; the remaining two for frame synchronization and signalling (control information).
The function of the time and space switch is to transfer information from a specified time slot on an incoming link to a specified time slot on an outgoing link. For a connection to be made, two types of switching operations must be performed. Switching from one link to another requires spatial translation (space switching). Switching from one time slot to another requires time translation (time switching). In a full-duplex connection between two arbitrary ports, each input time slot is paired with the same slot on the corresponding output link. Two connections must be made to allow information to be transferred in both directions between the two ports. It should be noted that 2N serial links and a total of 4 "port appearances" (4 time slots) for a connection are required using this switching scheme.
A multitude of architectural variations on time and space switching have been implemented. Various configurations such as time-space-time (TST) and space-time-space (STS) have been utilized, each with its own set of merits and drawbacks. However, all these variations still result in a minimum of 2N serial links and 4 port appearances. The reason for this result is that the digital switch matrix has always been considered a separate entity from the digital line interface to instruments. The purpose of a conventional switch matrix is to translate TDM channels from a set of input links to a separate set of output links; hence the need for 2N links. The conventional switch matrix is symmetrical; input and output formats are identical. The digital line interface circuitry in a conventional communication system then takes a given TDM output link and routes data from a specified time slot to a digital instrument via a twisted pair line. Similarly, the line interface circuitry also routes data from the digital instrument to a specified time slot on the corresponding TDM input link. Since the digital line interface is separate from the switch, the switch itself must be sized to allow for future growth in the number of ports in a system. This configuration is inefficient and adds cost to the system since more switching circuitry is present than is actually needed. A further disadvantage of the conventional switch matrix is that it is difficult to distribute portions of the switch to different printed circuit cards within the system. This is because every line interface port must be able to access every time slot of every input and output TDM link.
Elements of a central switch matrix have been fabricated in both NMOS and CMOS technology. The Thompson Semiconductor EF7331 and the SGS Semiconductor M088 are NMOS devices that switch 256 speech samples from 8 PCM in-going trunks to 256 speech samples from 8 PCM out-going trunks. Mitel's MT8980 is a CMOS device that performs the same function as the EF7331 and the M088. These time/space circuits are generally designed to be located together in the central equipment in one array. These devices do not offer any type of line interface capability.
Time slot selection circuitry has also been integrated. The Motorola MC14416 is a per-channel time slot assigner designed to provide receive and transmit time slot assignment for a PCM codec. This device performs the time division multiplexing function for a single codec on a single TDM link. Space switching is not performed. The device is intended to interface directly to a codec circuit on a line or trunk card and therefore does not include any twisted pair line interface circuitry such as an AMI encoder. The National TP3155 is similar in function to the Motorola MC14416, but slot assignment is performed for four separate codec circuits.
A digital line interface circuit has been integrated in CMOS technology. The Motorola MC145422 "UDLT" device transmits digital signals present at a codec-type interface over standard twisted pair phone wires using a "ping-pong" transmission scheme. The device services one port only and no switching functions are performed.
A digital line interface module has been built which provides an interface between a single twisted pair line and a single serial TDM data stream. The Mitel MH89700 device combines elements of time slot selection with a digital line interface but is intended for use with a central matrix switch. Space switching is not performed. Furthermore, the device is a hybrid module; not a single monolithic device.
A need has thus arisen for a distributed voice and data switching system complete with line interfaces for connections to digital telephones or data devices implemented in integrated circuit technology and which combines the switching and line interface functions in such a way as to minimize TDM links and their accompanying circuitry. A need has further arisen for a switching system in which the switching and line interface functions are combined to minimize port appearances, thereby reducing the system bandwidth requirements. A need has further arisen for a switching system which can be implemented on a per-port basis, thereby allowing for linear expansion of the system as ports are added.